Field adjusting apparatus for an electroplating bath

ABSTRACT

An electric field adjusting apparatus for adjusting electric field distribution inside an electroplating bath is provided. The electric field adjusting apparatus has a regulation plate with a plurality of evenly distributed through holes. A plurality of evenly distributed through holes with a smaller total through hole area is formed in the regulation plate that corresponds to an area of a plated film on a wafer that conducts a larger current during an electroplating process (close to the edge of the wafer). Meanwhile, a plurality of evenly distributed through holes with a larger total through hole area is formed in the regulation plate that corresponds to an area of the plated film on the wafer that conducts a smaller current during an electroplating process (close to the central region of the wafer). Furthermore, the total area of through holes around a particular location is inversely proportional to the current density in the plated film over the plating object.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the priority benefit of Taiwan application serial no. 91201976, filed Feb. 19, 2002.

BACKGROUND OF INVENTION

[0002] 1. Field of Invention

[0003] The present invention relates to an electric field adjusting apparatus for an electroplating bath. More particularly, the present invention relates to a method of adjusting electric field distribution inside an electroplating bath and an electric field adjusting apparatus doing the same.

[0004] 2. Description of Related Art

[0005] Electroplating is a process of coating a metallic film over a surface through electric dissociation of atoms. FIG. 1 is a rough sketch of a conventional electroplating bath. As shown in FIG. 1, a source metallic rod 150 is electrically connected to the anode of a power source 120. Meanwhile, a metallic substrate 140 to be electroplated is electrically connected to the cathode of the power source 120. Both the source metallic rod 150 and the metallic substrate 140 to be electroplated are immersed inside an electroplating bath with electroplating liquid 110. In general, the surface of the metallic substrate 140 must be processed and cleaned by acid to remove any oily substances thereon. After initiating the electroplating process by forming an electric circuit, atoms within the electroplating metal 150 from the anode 122 will dissolve in the electroplating liquid 110 in the form of positive metallic ions. These positive metallic ions will be attracted by the negative cathode 124 to migrate across the solution and ultimately neutralized by electric charges on the metallic substrate 140. Gradually, a layer of metallic material is built up on the surface of the metallic substrate 140.

[0006] The electroplating process is also used in the fabrication of semiconductor wafers. FIG. 2 is a schematic cross-sectional view (only the lower portion of an electroplating bath) of a conventional electroplating bath for electroplating wafers. As shown in FIG. 2, a wafer 240 (on a portion in cross-section is shown) fixed on a wafer holder 260 waiting to be electroplated is electrically connected to a cathode (not shown). A source metallic rod 250 fabricated from a metallic material including gold (Au), copper (Cu), aluminum (Al), titanium tungsten alloy (TiW), titanium (Ti) or chromium (Cr) is electrically connected to an anode (not shown). However, as shown in FIG. 2, the magnetic field is distributed non-uniformly with denser magnetic flux lines around the edges 242 of the wafer 240. Therefore, magnitude of the electric current flowing within this area is considerably higher. Ultimately, there will be a larger accumulation of electroplating material in these areas.

[0007]FIG. 3a is a diagram showing a conventional electroplating bath with a regulation plate therein for plating a metallic film over a wafer and FIG. 3b is a cross-sectional view of the electroplating bath in FIG. 3a. To improve-thickness uniformity of the coating due to an uneven current distribution around the edges 242 of the wafer 240, a circular perforation 272 is set up in the middle of a regulation plate 270 between the wafer 240 and the source metallic rod 250. The regulation plate 270 is able to adjust the electric field distribution of the wafer 240 inside the electroplating bath 200. In addition, there are a few paddles 230 for stirring the electrolyte 210 into a uniform mass. Here, the wafer 240 has a circular shape and hence the through hole 272 in the regulation plate 270 is also circular. FIG. 5 is a front view of a conventional regulation plate. As shown in FIG. 5, radius of the through hole in the regulation plate 270 varies according to the size of the wafer 240 to be electroplated. Typically, the regulation plate 270 is fabricated using a non-metallic material being non-conductive. FIG. 4 is a diagram showing the distribution of equipotential magnetic flux lines inside a conventional electroplating bath in a wafer plating process after inserting a regulation plate. As shown in FIG. 4, the convergence of magnetic flux lines around the edges 242 portion of the wafer 240 is improved and hence current over the wafer 140 is more uniformly distributed. In other words, variation of thickness of the coated metallic layer is greatly reduced.

[0008] Furthermore, electrode is mostly connected to the edge of a wafer. Therefore, if the source metallic rod 250 is fabricated from low resistant metallic material such as gold (Au) or copper (Cu), current distribution between wafer center and its edges is less likely to be affected by electrical resistance. In other words, a uniformly thick coating is formed throughout wafer surface after an electroplating process. However, if the source metallic rod 250 is fabricated from metallic material having a higher electrical resistance, including, for example, aluminum (Al), titanium-tungsten alloy (TiW) or chromium (Cr), electrical resistant from the edge to the center of the wafer 240 will be higher. Thus, even if a regulation plate 270 is inserted into the electroplating bath 200 between the wafer 240 and the source metallic rod 250, current distribution over the wafer 240 is unlikely to be uniform (edge current much higher than current close to the wafer center). Consequently, surface coating will be thicker around the edges than close to the center of the wafer.

SUMMARY OF INVENTION

[0009] Accordingly, one object of the present invention is to provide an electric field adjusting apparatus for an electroplating bath. The electric field adjusting apparatus has a regulation plate with a plurality of evenly distributed through holes instead of just one big through hole in the central region as in a conventional design. A plurality of evenly distributed through holes with a smaller total through hole area is formed in the regulation plate that corresponds to an area of a plated film on a wafer that conducts a larger current during an electroplating process (close to the edge of the wafer). Meanwhile, a plurality of evenly distributed through holes with a larger total through hole area is formed in the regulation plate that corresponds to an area of the plated film on the wafer that conducts a smaller current during an electroplating process (close to the central region of the wafer). Furthermore, the total area of through holes around a particular location is inversely proportional to the current density distribution of the plated film over the wafer. By adjusting the distribution of electric field over the wafer, variable current caused by a non-uniform distribution of electrical resistant within the wafer is minimized so that an electroplated film of uniform thickness is formed over the wafer after the electroplating process.

[0010] To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides an electric field adjusting apparatus for adjusting the electric field within an electroplating bath. The electroplating bath includes an anode and a cathode. To conduct an electroplating process, a wafer is electrically connected to the cathode and a source metallic rod is electrically connected to the anode. The wafer has a circular electroplating surface for plating a metallic film thereon. The electric field adjusting apparatus further includes a regulation plate inserted into the electroplating bath between the source metallic rod and the wafer. The regulation plate is set parallel to the wafer surface. The regulation plate has a plurality of hole regions. Each hole region has at least one through hole. Size of the through hole in each hole region is inversely proportional to the electric current density over the plating surface of the wafer.

[0011] The regulation plate is fabricated using a non-metallic material being non-conductive and the through holes in the regulation plate can be any shapes including, for example, a circular, an elliptical or a rectangular shape.

[0012] According to one aspect of this invention, a regulation plate with a variety of holes with sizes and number to match the current density flowing in a particular area of a plating object is formed. The total area of the through holes in a particular area of the regulation plate is inversely proportional to the current at the corresponding area on the plating object. Hence, the electric field over a plating surface is adjusted to reduce any non-uniformity in current distribution. Ultimately, an electroplated film with a uniform thickness is formed over the plating surface.

[0013] It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF DRAWINGS

[0014] The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings,

[0015]FIG. 1 is a rough sketch of a conventional electroplating bath;

[0016]FIG. 2 is a schematic cross-sectional view (only the lower portion of an electroplating bath) of a conventional electroplating bath with equipotential magnetic flux lines drawn during a wafer plating operation;

[0017]FIG. 3a is a diagram showing a conventional electroplating bath with a regulation plate therein for plating a metallic film over a wafer;

[0018]FIG. 3b is a cross-sectional view of the electroplating bath in FIG. 3a;

[0019]FIG. 4 is a diagram showing the distribution of equipotential magnetic flux lines inside a conventional electroplating bath in a wafer plating process after inserting a regulation plate;

[0020]FIG. 5 is a front view of a conventional regulation plate;

[0021]FIG. 6 is a schematic cross-sectional side view of an electroplating bath with all major internal elements according to this invention; and

[0022]FIG. 7 is a front view of the regulation plate to be used inside an electroplating bath according to this invention.

DETAILED DESCRIPTION

[0023] Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts.

[0024]FIG. 6 is a schematic cross-sectional side view of an electroplating bath with all major internal elements according to this invention. As shown in FIG. 6, the electroplating bath 300 includes a source metallic rod 350, an anode 322, a cathode 324, a power source 320, a plating object 340, some electrolytic solution 321, a plurality of paddles 330 and a regulation plate 370. The source metallic rod 350 is connected to the anode 322 of the power source 320. Both the source metallic rod 350 and the plating object 340 are immersed in a pool of electrolytic solution 310 inside the electroplating bath 300. The regulation plate 370 is fabricated using a non-metallic material being non-conductive. The regulation plate 370 is inserted in the space between the source metallic rod 350 and the plating object 340 such that the regulation plate 370 and the plating object 340 are parallel to each other. The paddles 330 are inserted into the space between the regulation plate 370 and the plating object 340 so that the electrolytic solution 310 is thoroughly mixed. The cathode 324 is normally clipped to the edge of the plating object 340. Hence, current on the metallic electroplated film 342 over the plating object 340 (the plating surface 341 of the plating object is circular) will flow in such a way that the rim area of the circular plating object 340 receive has the highest electric current but decrease towards the center.

[0025]FIG. 7 is a front view of the regulation plate to be used inside an electroplating bath according to this invention. As shown in FIG. 7, the regulation plate 370 has a plurality of through hole areas 372, 374 and 376. The through hole area 372 corresponds in position to the central area of the plating object 340 and contains only a single through hole 373. The through hole areas 374 and 376 contain a series of holes circularly enclosing the central through hole area 372. The edge of through hole 373 is centered upon the hole center. Each through hole areas 374 and 376 includes a multiple of circularly distributed through holes 375 and 377 (only two rows are shown). Size of the through holes 375 and 377 is smaller compared with the through hole 373. Furthermore, size of the through holes within the same row (375, 377) is identical but size of the through holes in a row (such as 377) further away from the center is smaller and more numerous than the through holes in a row (such as 375) closer to the center. The combined hole area of the through holes 377 is smaller than the combined hole area of the through holes 375. In addition, the ratio of total area between different through hole areas is inversely proportional to current distribution on the plating object 340. Similarly, the combined hole area of the through holes 375 is smaller than the hole area of the through hole 373 and the ratio of total areas between the two hole areas (374, 372) is also inversely proportional to the current distribution on the plating object 340. For example, in this embodiment, the number of through holes in the row of through holes 375 closest to the through hole 373 is 4 and the number of through holes in the row of through holes 377 further away from the center is 16. Comparing the two rows of through holes (375 and 377), more but smaller holes are found in rows further away from the center region. All the through holes including the central through hole 373 and the peripheral through holes 375, 377 are arranged to correspond in position to the plating area of the plating object 340.

[0026] In other words, the through holes on the regulation plate 370 are arranged in such a way that a larger number of small through holes are set up in the plate 370 facing the edge area of the circular plating object 340 where current density is highest a vice versa. That is, a smaller number of large through holes are set up closer to the central region of the plating object 340. Note that only one big through hole 373 is located at the very center of the regulation plate 370. With this setup, electric field on the plating object 340 is readjusted and any non-uniform resistance on some portion of the electroplating metallic film 342 during an electroplating process is smoothed out. Since differences in current distribution between the central and peripheral area of the plating object 340 are smoothed out, a uniform plated film 342 is ultimately obtained.

[0027] In this embodiment, the wafer has a circular plating surface and two rows (375 and 377) of through holes are formed on the regulation plate. In practice, the through holes need not be circular and the number of rows in the regulation plate need not be two. Furthermore, the number of through holes in each row is not limited to 4 for the row of through holes 375 and 16 for the row of through holes 377. In addition, shape of the through holes 373, 375 and 377 need not be circular, other shapes including a rectangular, a polygonal, an elliptical or some other special geometrical shapes are equally applicable. When the plating object 340 has a rectangular or polygonal plating surface 341 (not limited to circular), the same design rules according to this invention (that is: more small size through holes are punctured on the regulation plate in region corresponding to a larger current on the plating object and vice versa) can be used to fit the current distribution for that particular plating process.

[0028] Aside from plating the surface of a wafer, the field adjusting apparatus inside the electroplating bath can be used to plate other objects including, for example, a printed circuit board.

[0029] In conclusion, one major aspect of this invention is the design of a through hole pattern on a regulation plate and inserted into the space between a plating object and a metal source rod. The regulation plate is able to readjust the electric field pattern on the plating object and counteract some of the effect due to an uneven current distribution so that a uniformly thick electroplated layer is produced over the plating object.

[0030] It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents. 

1. An electric field adjusting apparatus inside an electroplating bath with an anode and a cathode therein, wherein the cathode is connected to a wafer, an source metallic rod is connected to the anode and the wafer has a circular plating surface, and the electroplating process includes forming a plated film on the plating surface, comprising: a regulation plate inside the electroplating bath in the space between the source metallic rod and the wafer with the regulation plate set parallel to the plating surface, wherein the regulation plate has a plurality of through hole areas with each through hole area having at least one through hole, total area of the through holes in each one of the through hole areas is inversely proportional to a distribution of current at the corresponding plated film on the plating surface.
 2. The electric field adjusting apparatus of claim 1, wherein the regulation plate is fabricated using a non-metallic material being non-conductive.
 3. The electric field adjusting apparatus of claim 1, wherein the through hole has a circular shape.
 4. The electric field adjusting apparatus of claim 1, wherein the through hole has a rectangular shape.
 5. The electric field adjusting apparatus of claim 1, wherein the through hole has an elliptical shape.
 6. An electric field adjusting apparatus inside an electroplating bath with an anode and a cathode therein, wherein the cathode is connected to a plating object, an source metallic rod is connected to the anode and the plating object has a circular plating surface, and the electroplating process includes forming a plated film on the surface of the plating object, comprising: a regulation plate inside the electroplating bath in the space between the source metallic rod and the plating object with the regulation plate set parallel to the plating surface of the plating object, wherein the regulation plate has a plurality of through hole areas with each through hole area having at least one through hole, total area of the through holes in each one of the through hole areas is inversely proportional to the distribution of current at the corresponding plated film on the plating surface.
 7. The electric field adjusting apparatus of claim 6, wherein the regulation plate is fabricated using a non-metallic material being non-conductive.
 8. The electric field adjusting apparatus of claim 6, wherein the through hole has a circular shape.
 9. The electric field adjusting apparatus of claim 6, wherein the through hole has a rectangular shape.
 10. The electric field adjusting apparatus of claim 6, wherein the through hole has an elliptical shape.
 11. An electroplating apparatus, comprising: an electroplating bath; a pool of electrolytic solution inside the electroplating bath; a power source; an anode connected to the power source; a cathode connected to the power source; a plating object connected to the anode and immersed in the pool of electrolytic solution inside the electroplating bath, wherein the plating object has a plating surface and the electroplating process includes plating a metallic film over the plating surface of the object; a source metal rod connected to the cathode and immersed in the pool of electrolytic solution inside the electroplating bath; and a regulation plate inside the electroplating bath in the space between the source metallic rod and the plating object with the regulation plate set parallel to the plating surface, wherein the regulation plate has a plurality of through hole areas with each through hole area having at least one through hole, total area of the through holesin each one of the through hole areas is inversely proportional to the distribution of current at the corresponding plated film on the plating surface.
 12. The electroplating apparatus of claim 11, wherein the regulation plate is fabricated using anon-metallic material being non-conductive.
 13. The electroplating apparatus of claim 11, wherein the plating object includes a printed circuit board.
 14. The electroplating apparatus of claim 11, wherein the plating object includes a wafer.
 15. The electroplating apparatus of claim 11, wherein the plating surface is circular.
 16. The electroplating apparatus of claim 11, wherein the plating surface is rectangular.
 17. The electroplating apparatus of claim 11, wherein the plating surface is polygonal.
 18. The electroplating apparatus of claim 11, wherein the apparatus further includes a paddle inserted in the space between the regulation plate and the plating object and immersed in the pool of electrolytic solution for mixing the electrolytic solution thoroughly.
 19. The electroplating apparatus of claim 11, wherein the through hole has a circular shape.
 20. The electroplating apparatus of claim 11, wherein the through hole has a rectangular shape.
 21. The electroplating apparatus of claim 11, wherein the through hole has an elliptical shape. 